Method for optimizing lacquers

ABSTRACT

The present invention relates to a method of and a device for optimizing at least one coating material at at least one point of a substrate surface to which the coating material is applied. The method, which is carried out with the corresponding device, comprises at least the following steps: a) applying said at least one coating material to said at least one point of the substrate surface, b) curing said at least one coating material at said at least one point of the substrate surface, and c) determining the state, especially the curing and/or yellowing and/or gloss, of said coating material at said at least one point of the substrate surface, possessed by said coating material as a consequence of steps a) and b).

The present invention relates to a method of and to a device foroptimizing coating materials, especially radiation-curable coatingmaterials.

coating materials, especially radiation-curable coating materials,generally have a highly complex composition. Key components of aradiation-curable coating material include reactive diluents, oligomers,prepolymers, synergists, photoinitiators, light stabilizers, such as UVabsorbers or sterically hindered amines, for example, pigments, dullingagents, flow agents and other additives. This results in a greatdiversity of possible coating compositions. To date, the coatingmaterials have been formulated in practice by the trial and errorprinciple and with many years of experience have been optimized inlaborious series of tests, which have to be analyzed manually. Despitethis costly and time-consuming procedure, the large number of possiblecoating compositions throws up only random hits of adequatelysatisfactory quality but does not lead to high-quality coatingsdetermined systematically and in a targeted manner, since targetedinvestigation of the abovementioned sphere of parameters, conducted inparallel, is impossible owing to the massive effort it would entail.Predicting the properties of a certain composition for a coatingmaterial is possible only to a limited extent, since various components,such as photoinitiators and UV stabilizers, for example, have effects onone another which are nonlinear.

It is an object of the present invention to provide a method and adevice for optimizing coating materials, said method and device allowingtargeted and systematic variation in the key components of a coatingmaterial, especially of a radiation-curable coating material, in orderto be able to arrive objectively at an optimum composition of thevarious components of the coating material.

We have found that this object is achieved by the method of theinvention, as claimed in claim 1, and by the corresponding devices, asclaimed in claims 5, 9 and 10 and the dependent subclaims. The method ofthe invention constitutes a method of optimizing at least one coatingmaterial at at least one point of a substrate surface to which thecoating material is applied. In accordance with the invention, at leastthe following steps of the method are conducted in a device provided forthat purpose:

a) applying said at least one coating material to said at least onepoint of said substrate surface.

Preferably, two or more different coating compositions are appliedsimultaneously at different points of the substrate surface, whichtogether form a grid. The different compositions are suitably applied tothe corresponding points of a desired substrate surface, such as a woodor metal or paper surface, for example, with the aid, for example, ofmetering pipettes, microdoctors or microspray heads, preferably undercomputer control.

The points of the substrate surface at which each of the differentcoating compositions is applied are preferably chosen to be very small,so as to enable application of many different coating compositions to asingle substrate surface. The points of the substrate surface at whichthe coating compositions are applied preferably form a kind of matrix,corresponding to an arrangement of m rows each having n columns, n and meach being less than 1000. The size of an individual point of thesubstrate surface to which one of the different coating compositions isapplied depends primarily on how the coating material is later to becharacterized. Using current techniques it is possible to investigate upto 10,000 different coating compositions on 10 cm² of a substratesurface.

Thereafter, the coating compositions are optionally dried in order, forexample, to allow evaporation of the solvent which is required incertain cases for optimum mixing.

b) Curing said at least one coating material at said at least one pointof said substrate surface.

Preferably, the coating material, or the different coating compositionsapplied at different points of the substrate surface, which togetherform a grid, are radiation-cured. In the course of radiation curing,described for example in J.-P. Fouassier, Photoinitiation,Photopolymerization and Photocuring, Hanser Publishers, Munich, 1995,the mixture of the individual components of a coating composition isconverted by exposure, preferably UV exposure, into a three-dimensional,mechanically stable polymer network. Advantages of this technology liein the high speed, low energy consumption, virtually complete absence ofenvironmentally harmful reaction products on curing, and low costs.Curing is preferably performed simultaneously for all correspondingpoints of the substrate surface, preferably by means of exposure over alarge area with UV light or with electron beams. This results inthree-dimensionally cured coating films at the corresponding points ofthe substrate surface. Exposure over a large area is very economic intime and energy terms and, furthermore, provides the required uniformprocessing of all coating films applied to the substrate surface.Preferably, the coating material, or the different coating compositionsapplied at different points of the substrate surface, which togetherform a grid, is or are heated in the course of curing. In this way it ispossible first to accelerate the reaction—i.e., the formation of thethree-dimensional network—and, second, to ensure that the reactionproceeds completely by itself.

c) Determining the condition, especially the curing and/or yellowingand/or gloss, of said at least one coating material at said at least onepoint of said substrate surface, possessed by said coating material as aconsequence of steps a) and b).

As set out above, it is possible according to the invention to determineand/or analyze individually each of the parameters describing thecondition, such as curing, yellowing and gloss, for example, or else allof the parameters are determined and/or analyzed, preference being givento the determination/analysis of all parameters since it gives virtuallya complete picture of the condition of the coating material.

The cured coating material, or the different cured coating compositionsapplied to the substrate surface, is or are preferably characterized bymeans of a spectroscopic technique which has a high lateral localresolution and, if required, a sufficient depth resolution. In this wayit can be ensured that in each case only one coating composition ischaracterized at one of the relevant points of the substrate surface,without any interaction with coating compositions that have been appliedat adjacent points on the substrate surface. It is preferable here touse the method of confocal Raman spectroscopy. In this method, thecoating network which forms in the course of curing is detected on thebasis of the disappearance of reactive groups; in other words, thereaction conversion which takes place in the course of curing isdetermined directly (W. Schrof, L. HäuBling, Tiefenauflösung derTrocknungsvorgänge in Lackfilmen, in “Farbe und Lack”, 103, 1997,22-27). In this case, by using highly sensitive spectrometers whichoperate primarily in backscattered light, the measurement times can beshortened to the order of seconds. For the state of the art in the fieldof Raman spectroscopy and, respectively, confocal imaging, reference maybe made to Schrader B., Infrared and Raman Spectroscopy, VCH, Weinheim,1995 and Markwort L., Kip B., Da Silva E., Roussel B., Appl. Spectrosc.49 (1995) 1411-30. In addition to confocal Raman spectroscopy it is alsopossible to use IR spectroscopy or fluorescence spectroscopy.Fluorescence methods (O. Wolfbeiβ, Fluorescence Spectroscopy: NewMethods and Applications, Springer, Berlin, 1993) analyze the structureof the coating network formed as a result of curing, analysis beingcarried out on the basis of the decrease in local mobility ortranslation diffusion of fluorescence probes. All of these opticalmethods can be carried out with high local resolution, in combination,for example, with suitable lenses or with a microscope. In a furtherpreferred embodiment the optical imaging operations are carried out notwith lenses or microscopes but with optical fibers (E.-G. Neumann,“Single Mode Fibres”, Springer, Berlin, 1988). As already mentioned,depth profiles of the network of the coating material to becharacterized, said network coming about as a result of curing, can bedetermined by means of confocal Raman spectroscopy. Consequently,additional information is obtained relating to typical phenomena inradiation curing, such as, for example, oxygen inhibition at thesubstrate surface, or inadequate curing in deeper regions owing todepth-of-penetration effects for the UV light. An additional selectionof suitable coating compositions is therefore made possible. By means ofthe confocal setup, for example, with a confocal diaphragm in thedetection beam path, a depth level with a thickness of down to 1 μm isselected. An additional automatic focusing unit, which is preferablyused, enables imaging onto the coating surface. Depth profiles areobtained by subsequent measurements at planes deeper relative to thecoating surface. This is achieved preferably by computer-controlledraising of the substrate on whose surface the coating material has beenapplied, or by lowering the focus by means, for example, of apiezo-controlled optic.

In a further embodiment in accordance with the invention, curing is alsodetermined mechanically by means of micro-sized hardness measuringdevices (H.-H. Behncke, W. Weiler, Computergesteuerte Mikrohärtemessungunter Prüfkraft, in “Materialprüfung”, Vol. 7, 1988) such as, forexample, a Fischerscope or a nanoindenter. Penetration measurements atindividual points (Bernham, R. J. Colton, Measuring the NanomechanicalProperties and Surface Forces of Materials Using an Atomic ForceMicroscope, J. Vac. Sci. Technol. Vol. A7, 1989, 2906) also make itpossible to determine mechanical properties of the coating materialconcerned. With the aid of suitably chosen displacement platforms forthe corresponding experimental setup, it is possible to scan the entiresubstrate surface.

The present invention additionally provides a device for the gridlikeapplication of at least one coating material to a substrate surface,comprising:

a) a metering means for metering at least one component and preferablyall components of said at least one coating material.

In this case the metering of the at least one component and preferablyall components of said at least one coating material preferably takesplace automatically.

b) A mixing means for mixing the individual components of said at leastone coating material.

There are a number of possible procedures for achieving thorough mixingof the individual components. The components of a coating compositioncan be dissolved in a common solvent or in different, mutuallycompatible solvents and so mixed with one another, and/or they can beheated, and/or they can be mixed mechanically, such as by stirring orthe use of ultrasound, for example. The systematic variation of thecomponents and of their concentration in predeterminable steps producesa large number of different, liquid coating compositions.

c) A pipetting or spraying means for successive application of said atleast one coating material to points on the substrate surface whichtogether form a grid and which are locally mutually delimitable.

Said at least one coating material, or the different coatingcompositions, is or are applied with the aid of pipetting or sprayingmeans to points on the substrate surface which are provided for thispurpose. Preferably, use is made of pipetting or spraying robots, whichpermit an automated procedure which is therefore economic in terms oftime and cost.

The locally delimitable points on the substrate surface, togetherforming a grid, which are intended for the application of in each caseone coating composition, correspond, in a preferred embodiment of theinvention, to recesses in the substrate surface which in their entiretyform a grid on the substrate surface. The corresponding coatingcompositions are then filled into these recesses.

In a further preferred embodiment, the substrate surface is modifiedhydrophilically or, respectively, hydrophobically in a suitable manner,so that, here again, coating compositions can be applied to thesubstrate surface in a grid format without the unwanted mixing ofdifferent coating compositions.

To avoid sublimation of volatile components such as reactive diluents,the coated substrate is preferably covered with a UV-permeable film orcoat.

In a further preferred embodiment of the invention, a grid of coatingcompositions is likewise applied to the substrate surface, but in thiscase the individual components are brought together directly on thesubstrate surface by means of suitable pipetting means, preferablypipetting robots, or by means of droplet generators, rather than beingbrought together first in a separate vessel. In order to be able toensure thorough mixing of the individual components of a coatingcomposition, they are always applied only in part, alternatingly orsimultaneously, i.e., for example, by applying picoliter or nanoliterdroplets. The reduction of the droplet size to a diameter in themicrometer range or to a volume in the picoliter range permits thoroughmixing in the case of alternate or simultaneous application. Thistechnique circumvents the step of mixing the individual components of acoating composition in external vessels. The possibility of employingthis method, however, depends very heavily on the nature and interactionof the individual components of the corresponding coating compositionthat are to be mixed.

Furthermore, the present invention also provides a device for optimizingat least one coating material on a substrate surface, said devicecomprising, in addition to the device already described for the gridlikeapplication of at least one coating material to a substrate surface, atleast one curing means, preferably for simultaneous curing and, inparticular, for simultaneous radiation-curing of said at least onecoating material at the locally mutually delimitable points, whichtogether form a grid, on the substrate surface, and at least one meansof determining—preferably simultaneously—the condition of said at leastone coating material at the locally mutually delimitable points, whichtogether form a grid, on the substrate surface.

Determining the condition of said at least one coating material entailsprimarily determining the curing. In the case of the radiation curing ofa coating composition, the curing is determined essentially by theconversion of the reactive components. As already mentioned, thisconversion can be determined with the aid of spectroscopic methods. Thevibration-based spectroscopic techniques, such as Raman and IRspectroscopy, determine the reaction conversion directly. The use ofRaman spectroscopy is preferred. For this purpose, the device comprisesa means of irradiating monochromatic light to at least one of the pointson the substrate surface which together form a grid and are locallydelimitable with respect to one another, and of detecting scatteredlight from said at least one of the points on the substrate surfacewhich together form a grid and are locally delimitable with respect toone another, in order to be able thus to determine the reactionconversion, as a consequence of radiation curing, at said at least oneof the points on the substrate surface which together form a grid andare locally delimitable with respect to one another.

Another alternative is offered by fluorescence spectroscopy, in whichthe structure of the physical network is determined by means of dopedprobes.

In order to be able to investigate relatively large lateral regions ofthe substrate surface, which is covered in gridlike manner with at leastone but preferably two or more different coating compositions, it isalso possible, within the framework of the present invention, to movethe substrate (covered with the coating compositions to be analyzed)automatically by means of computer-controlled displacement platformssuch that each of the points on the substrate surface which togetherform a grid and are locally delimitable with respect to one another,each of which points is covered with a coating composition, can beanalyzed in succession. This technique permits parallelized measurementswith a high sample throughput within a short time and without a highlevel of personnel deployment.

Determining the condition of the coating material also entails, interalia, the characterization of the gloss and/or yellowing of the coatingcompositions to be investigated. As with the determination of thecuring, gloss and yellowing can also be determined with local resolutionfrom a spectroscopic analysis of reflected and/or scattered light.

Further advantages, features and possible applications of the inventionare evident from the description which follows of an experimental setupaccording to the invention in conjunction with the corresponding figure,in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a diagrammatic setup of a device, according to theinvention, for optimizing at least one coating material on a substratesurface.

In FIG. 1, the setup of a device of the invention is showndiagrammatically. Coating compositions to be investigated are eachapplied to the locally mutually delimitable points 2, which togetherform a grid, on a substrate surface 1. Using a spectroscopic technique,in this case the curing, gloss and yellowing of the individual coatingcompositions applied to the various points 2 of the substrate surface 1,and already cured, are determined. At these corresponding points, aspectrum is recorded in each case by means of an imaging and focusinglens 3 and a spectrometer 4: for example, a Raman microscope. From thespectra obtained in this way, it is then possible to make specificstatements about the reaction conversion at the corresponding points ofthe substrate surface and thus, ultimately, about the curing, gloss andyellowing of the coating compositions applied at those points, that arethe subject of investigation. Preferably, the reaction conversion isdetermined by means of an optical system which makes it possible toshift the focus of the monochromatic light introduced within the appliedcoating film, such as, for example, by means of confocal Ramanspectroscopy. In this context it is possible to adjust an arbitrarydepth segment within the coating film by moving the transmitter or themicroscope lens back and forward along the optical axis by way of apiezoelectric translator. It is possible to achieve an accuracy of fromabout 1 to 3 μm in respect of the adjusted focus, i.e., of the desireddepth segment. The depth definition can be adjusted by using a diaphragmor optical fiber of suitable internal diameter. In order to be able toinvestigate the entire substrate surface, i.e., in both the x and ydirections, it is possible to move the substrate by means of acontrollable translation platform.

We claim:
 1. A process for automatically producing and characterizing aplurality of coating compositions on a substrate surface, comprising a)applying different radiation-curable coating compositions at differentpoints of a substrate surface, which together form a grid by means ofmetering pipettes, microdoctors or microspray heads under computercontrol, b) curing the coating compositions by radiation, and c)characterizing the different radiation cured coating compositions on thesubstrate surface are by means of spectroscopic methods, selected fromthe group consisting of confocal Raman spectroscopy, IR and fluorescencespectroscopy, and/or by means of microsized hardness measuring devices.2. The process of claim 1, wherein the points, which together form agrid, on the substrate surface are recesses in the substrate surfaceinto which the coating compositions are introduced.
 3. The process ofclaim 2, wherein the substrate surface is modified hydrophilically orhydrophobically, so that coating compositions can be applied to thesubstrate surface in a grid format without the unwanted mixing ofdifferent coating compositions.
 4. The process of claim 1, wherein theindividual components of the coating compositions are brought togetherdirectly on the substrate surface by means of droplet generators.
 5. Theprocess of claim 1, wherein all coating compositions are simultaneouslycured by means of exposure over a large area with UV-light or withelectron beams.
 6. The process of claim 1, wherein the cured coatingcompositions are characterized by means of confocal Raman spectroscopy.7. The process of claim 1, wherein the cured coating compositions arecharacterized by means of a fisherscope or a nanoindenter.
 8. Theprocess of claim 1, wherein the substrate is moved automatically bymeans of a computer-controlled displacement platform such that each ofthe points on the substrate surface which together form a grid, each ofwhich points is covered with a coating composition, are analyzed insuccession.
 9. The process of claim 1, wherein the coating compositionsare characterized with respect to curing, gloss and yellowing.
 10. Theprocess of claim 1, wherein the different coating compositions aresystematically varied, in order to arrive at an optimum composition.